This invention relates to a process for recovering C3+ hydrocarbon compounds from crude carbon dioxide fluid comprising C1+ hydrocarbon compounds and at least one non-hydrocarbon “heavy” impurity. The invention has particular application in processing crude carbon dioxide gas for recycle in a carbon dioxide enhanced oil recovery (EOR) process for extracting crude oil.
In a carbon dioxide EOR process, carbon dioxide is injected into aging oil fields to extract more oil than is possible by conventional water-flooding. As the oil is extracted from the field, carbon dioxide that is dissolved in the oil is recovered, recompressed, and recycled to the oil field. The recycled carbon dioxide can contain a significant amount of natural gas liquids (NGLs). The value of the NGL in the recycled carbon dioxide can reach levels where it is economically viable to recover these hydrocarbons for sale.
Recovering hydrocarbons from crude carbon dioxide obtained from an EOR process and recycling the carbon dioxide to the EOR process is known generally. Examples of previous attempts to develop suitable processes include U.S. Pat. No. 4,753,666A (Pastor et al; 1988) which discloses distilling a hydrocarbon-rich carbon dioxide gas in a single column to produce an NGL (C4+) stream and an overhead stream containing substantially all of the carbon dioxide, methane, ethane, propane and hydrogen sulfide. An external propane refrigeration system is used for the condenser and a bottom reboiler maintains the bottom temperature at about 360° F. (182° C.). This process can be inefficient and expensive due to the use of the external refrigeration system. In addition, the valuable propane component is also lost in the process and is reinjected with the carbon dioxide recycle stream. Further, there is a relatively high duty requirement on the bottom reboiler, which is provided by a hot oil system or steam.
Another example is disclosed in WO2010/076282A (Marsh; 2010). In this process, a Joule-Thomson valve and a low temperature separator are placed before a distillation column in order to cool, condense and separate the heavier components (C5+) from the associated gas. The C5-C6 hydrocarbons are then selectively recovered from the bottom of the distillation column. Carbon dioxide containing C1-C4 hydrocarbons is recovered from the top of the separator and from the top of the distillation column and sent back to the injection wells. There is a reboiler placed at the bottom of the distillation column. A disadvantage of this process is that valuable propane and butane components are not recovered and are recycled with the carbon dioxide stream. In addition, the entire feedstock is compressed before it goes through the Joule-Thomson valve so this process has a high power consumption. There is also a relatively high duty requirement on the bottom reboiler.
U.S. Pat. No. 8,505,332A (Prim et al; 2013) and its continuation application, US2011/00197629A, describe a two-stage distillation process that separates a hydrocarbon-rich carbon dioxide stream recycled in the EOR process into a purified carbon dioxide gas, a heavy NGL (C4+) stream and a light NGL (C3-C4) stream. The first distillation column separates C3+ from the associated gas with 20-35% C3 recovery rate. The bottom liquid is sent to an amine unit to remove the acid gas(es) remaining in the NGL. The purified NGL is then sent to the second column to separate C3-C4 from the heavier components. The process is, however, inefficient and expensive in view of the use of an external refrigeration system. In addition, an external solvent is used for acid gas removal. There is also a relatively high duty requirement on the bottom reboiler, provided by a hot oil system.
It is desirable to develop a process for recovering C3+ hydrocarbons from a crude carbon dioxide recycle stream from an EOR process, that is simple, efficient, and capable of optimizing overall hydrocarbon production with reduced capital and operating costs.